1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, and the like, and more particularly to an image forming apparatus that develops a latent image on an image bearing member by a developing device including a developer bearing member to carry developer borne on a surface thereof by magnetic force of a magnet included therein to a position to develop the latent image on the image bearing member, a limiting member to limit a thickness of a layer of the developer on the developer bearing member and a developing device casing.
2. Discussion of the Background
In an image forming apparatus, it is known that magnetizable developer such as a two-component developer composed of magnetizable carrier and toner, or one-component developer composed of only magnetizable toner may be used as a developer, and frictional electricity of the magnetizable developer borne on a developing roller, as a developer bearing member, including a magnet is accelerated by employing a magnetizable limiting member to limit the thickness of the developer borne on the developing roller.
As such a magnetizable limiting member, there is known one which is formed of only a magnetizable member, and another which is formed of a non-magnetizable member and which includes a magnetizable member fixed thereupon, with the distal edge thereof facing the developing roller being aligned with that of the non-magnetizable member.
Further, there is also known an image forming apparatus having a rotary developing apparatus in which a plurality of developing devices, each including a magnetizable limiting member or a limiting member with a magnetizable tip-end, are disposed around a rotating shaft of the rotary developing apparatus. A latent image on an image bearing member is developed by moving the predetermined developing device to a developing station facing the image bearing member by rotating the rotary developing apparatus and by carrying developer borne on a surface of a developing roller of the developing device to a position to apply the developer to the image bearing member.
A latent image of each color of an image which is formed on the image bearing member is developed by a corresponding developing device of the aforementioned rotary developing apparatus and the developed image is transferred onto an intermediate transfer member. A full color image is formed by individually transferring each color toner image thus developed one after another onto the intermediate transfer member while superimposing one above the other.
In the above-described developing devices, the length of a magnet included inside the developing roller is generally made shorter than that of the developing roller and is positioned substantially in the middle of the developing roller, i.e., the lengthwise center of the magnet is aligned with that of the developing roller. A density of magnetic force lines at two ends of the magnet is greater than that at the middle of the magnet by diffraction of the magnetic force lines, and the magnetic force at the surface of the developing roller facing the two ends of the magnet is therefore stronger than that at the middle part of the developing roller. As a result, the amount of developer attracted to the surface of the developing roller becomes greater at the part of the developing roller facing the two ends of the magnet than at the part facing the middle part of the magnet, and accordingly, an excessive amount of the developer collects on the surface of the developing roller at the part facing the two ends of the magnet.
Because the excessive amount of the developer has fluidity to some extent, the developer at the parts of the developing roller facing the two ends of the magnet tends to move in a lateral direction towards ends of the developing roller in the axial direction of the developing roller (hereinafter referred to as two ends of the developing roller) from the parts of the developing roller facing the two ends of the magnet. On the other hand, both end parts of the developing roller not facing the magnet do not receive an effect of a magnetic force of the magnet. Therefore, if the developer is moved by the aforementioned lateral movement of the developer to the end parts of the developing roller not facing the magnet, the developer cannot be supported on the surface of the developing roller.
The developer moved to the end parts of the surface of the developing roller where the magnetic force is not exerted is immediately conveyed towards the image bearing member by a rotation of the developing roller, through an opening of the developing device casing and causes various problems as described below.
For example, even though the aforementioned developer carried to the developing position is further carried toward the inside of the developing device along with the rotation of the developing roller, a part of the developer cannot enter the clearance between an edge portion of the aforementioned opening and the surface of the developing roller and falls from the surface of the developing roller. Such falling phenomenon of the developer is called "developer overflow" and causes problems such that an internal part of the image forming apparatus is soiled with the developer, the developer adheres on a non-image part of the image bearing member (particularly, two end parts of the image bearing member) and the developer soils a recording member.
Further, the developer carried to the aforementioned developing position sometimes melts by being pressed between the image bearing member and the developing roller and is adhered to a surface of the image bearing member or the surface of the developing roller. Furthermore, when such melting and adhering phenomenon of the developer occur, various problems, such as, for example, abrasion of a surface layer of the image bearing member with the developer adhered to the developing roller, fouling of a background of a recording member with the developer adhered to the image bearing member, and insufficient cleaning effect for the image bearing member, are caused.
In addition, when excessive melting and adhering of the developer occurs, the frictional resistance between the limiting member and the developer on the developing roller, which is melted and adhered to each other is increased, and thereby torque required to rotate the developing roller becomes uneven. The unevenness of the torque causes unevenness of the rotation speed of the developing roller, resulting in a stripe image having a periodic unevenness of the density of the image.
In particular, in an image forming apparatus employing the above-mentioned rotary developing apparatus including a plurality of developing devices, when the developing apparatus rotates, the developing devices have postures different from each other. As illustrated in FIG. 2, when one developing device is in the developing station facing the image bearing member, another developing device that will be moved to the developing station next has a posture in which the developer is moved to the developing roller from the position above the developing roller and accordingly, a large amount of the developer is moved onto the developing roller by gravity. When the developer is moved by gravity to the developing roller, the frictional force between the developer and the surface of developing roller and an attractive force between the developer particles is small relative to the frictional force between the developer when the developer remains on the surface of the developing roller by being attracted by the magnetic force. Therefore, the developer being moved by gravity onto the developing roller easily moves in the lateral direction. Accordingly, in the image forming apparatus employing the rotary developing apparatus, the various problems as mentioned above also occur.
For solving the above-mentioned various problems, in Japanese Laid-Open Patent Publication No. 9-265238, a developing device having a magnetizable limiting member configured such that thickness or height at two ends thereof is greater than that at a middle part thereof is proposed. Further, in Japanese Laid-Open Patent Publication No. 8-202153, there is proposed a developing device in which a sealing member made of a magnetizable member is disposed adjacent to side faces of two ends of a limiting member in a longitudinal direction, and the magnetizable limiting member is disposed at a position approximately facing a magnet in a developing roller in a circumferential direction of the developing roller. The developing device thereby strengthens a concentration of the magnetic force lines between the limiting member and the developing roller at two end parts of the developing roller relative to the middle part of the developing roller in a longitudinal direction. That is, these developing devices have a magnetizable limiting member that is thicker at the two end portions facing end parts of the developing roller than at the other parts, or has protruding portions, so as to increase the magnetic force for the two end parts of the developing roller. Accordingly, the supporting force for the developer borne at the two end parts of the developing roller is strengthened and the developer is prevented from being conveyed towards the image bearing member through an opening of the developing device casing by a rotation of the developing roller.
However, in the developing device that employs the magnetizable limiting member formed of only a magnetizable member, or a limiting member which is formed of a non-magnetizable member and which includes a magnetizable member fixed thereupon with the distal edge thereof facing the developing roller being aligned with that of the non-magnetizable member, there is a shortcoming that an amount of the developer passing through a developing doctor gap between a distal edge of the limiting member and the surface of the developing roller fluctuates according to variations of the developing doctor gap which may be caused by assembly errors and so forth.
Further, in the developing devices having the magnetizable limiting member or a non-magnetizable member with a magnetizable member fixed thereupon at the distal edge portion thereof as described above, when a distance between the limiting member and the developing roller is set so that a limited amount of the developer required for forming an image can be obtained, the magnetic force of the limiting member excessively affects the developer on the surface of the developing roller, resulting in application of excessive stress on the developer. The excessive stress deteriorates characteristics of the developer. More specifically, when two-component developer composed of magnetizable carrier and toner is used, a charging capacity of the magnetizable carrier is significantly decreased due to wear of coating of the carrier and, as a result, an expected life of the developer cannot be obtained.